Industrial refrigeration in the low temperature region down to about -110.degree. C. has become very widespread in recent years. A considerable proportion of this development has taken place in the field of laboratory equipment, Cryostatic temperature controls and testing chambers for the testing of materials at low temperatures. Refrigeration plants for the aforesaid temperature regions are also required to an increasing extent in medicine, chemistry and processing technology as well as steel refining and need to be designed for higher outputs.
The lubricants used in refrigeration plants have for a long time been simple mineral oils or ordinary naphthenic oils but these are unsatisfactory in many respects (unresistant to extremes of temperature and subject to wide fluctuations in viscosity).
Semi-synthetic oils, fully synthetic alkyl aromatic compounds and poly-.alpha.-olefines have brought further improvements. Synthetic liquid lubricants on a chemical basis very different from mineral oils, e.g. butyl esters of polysilicic acids, have been the result of this development. Liquid lubricants of this type, however, have a certain susceptibility to hydrolysis which may cause them to gel in the presence of water.
To use organopolysiloxanes as lubricants would be one solution to the problem of hydrolysis but it is well known that, for example, dimethylpolysiloxanes are poor lubricants even though their relationship of viscosity to temperature could afford some advantages for a liquid lubricant in low temperature installations.
Siloxanes which are substituted with phenyl, halophenyl or trifluoropropyl groups have good lubricating properties but poor viscosity/temperature relationships compared with dimethylpolysiloxane (see J. of Chem. and Engng. Data 6 (1961), 155). Although such siloxanes have been used for the preparation of high temperature lubricating fats, they have not been used as low temperature lubricants.
Lubricants which are stable over a very wide temperature range have also been disclosed in U.S. Pat. No. 3,642,626. These lubricants are polymeric fluorosilicones with a relatively high degree of polmyerization but they provide no advantages as low temperature lubricants due to their high viscosity at low temperatures. Also their preparation is complicated and costly.
To overcome this problem, a fluorine-containing siloxane of the general formula EQU R.sub.F R.sub.2 SiOSiR.sub.2 R.sub.F
in which R.sub.F denotes the group C.sub.n F.sub.2n+1 --CH.sub.2 --CH.sub.2 -- (n=1-12) has been proposed in German Pat. No. 2,750,980.
This oil may be used alone or as a mixture with known lubricants.
When this oil was subjected to technical tests, it was found to have advantages both for the preparation of low temperature lubricants and for the preparation of fats according to German Pat. No. 1,769,094. This low temperature lubricant has, however, low initial viscosity which renders it unsuitable for certain purposes for which it has been found necessary to use lubricating oils with viscosities preferablyin the class according to ISO VG 46. This means that these oils are required to have kinematic viscosities of from 41.multidot.4 to 50.multidot.6 mm.sup.2 /sec at 40.degree. C.